This invention relates to forming apparatus and more particularly to a pultrusion apparatus for forming shapes of reinforced plastic, wherein it is desired to fabricate a shape having a thick cross-section.
A review of a portion of the prior art discloses, in U.S. Pat. No. 2,871,911 issued to Goldsworthy et al., a resin reinforced glass fiber production apparatus with high frequency (radio frequency) heating of the shaping die, and (optional) external heating of the hardened roving by gas burners (Column 4).
U.S. Pat. No. 3,674,601 issued also to Goldsworthy discloses a pultrusion apparatus which has radio frequency (RF) heating and may have an externally heated finish die (claim 7). The finished stock can be virtually of any profile and any distribution of mass throughout the profile (Column 2).
U.S. Pat. No. 3,684,622 also issue to Goldsworthy discloses a pultrusion apparatus which has an (unheated) torpedo for introduction of resin into the interior zone of the composing tube (Column 7). RF or microwave heating is used.
None of these patents address the problem, when it is desired to pultrude a part having a particularly thick cross-section, of the internal cracking that results due to thermal stresses induced by uneven heating and cooling of the interior of the part relative to the exterior of the part during the gelling, and subsequent exothermic curing of the part.
A method and apparatus, therefore, need to be developed which will solve the problem of internal stress cracking due to the existence of an undesirable thermal gradient during the pultrusion process. Such a method and apparatus should blend into existing pultrusion process methods and equipment in order to allow manufacturers to increase the variety of pultruded cross-sections available to the public. More specifically, the method and apparatus of the current invention should allow the pultrusion of elongated articles having a cross-section greater than approximately 1" in diameter.